[Variability in Toxoplasma virulence depending on the sensitivity of the macroorganizm]

The conditions of the increase in the virulence of toxoplasma lines of the Czech low-virulent strain obtained by means of the infection of mice or chicken embryos with one cyst washed three times in physiological solution were investigated. It has been shown that all the 12 were potentially virulent since in the process of frequent (in each 7 days) passage through sensitive animals (bredless mice) they increased their virulence. The lines of toxoplasms passed only one time through chicken embryos increased their virulence quicker (1 to 10 passages) than lines passed only through mice (18 to 22 passages). At the simultaneous passing of toxoplasms of the same line through sensitive animals (bredless mice) and through more resistant ones [mice of lines AKR and BALB/c] toxoplasms increased their virulence during the 4th passage and did not change it up to the 25th passage, respectively.